Molecular sieve abstract
The present invention provides a molecular sieve composite membrane,
which includes an anodic alumina membrane as a support and the uni-directionally
oriented molecular sieve membrane grown in situ on the anodic alumina
membrane. The close packing transitional metal containing aluminophosphate
AFI molecular sieve crystals have successfully been grown on the
anodic alumina. The molecular sieve phase bounded strongly and anchored
into the anodic alumina membrane. Besides, the specific cylindrical
channels of the anodic alumina membrane provides the template function
to orient the growth of molecular sieves.
Molecular sieve claims
What is claimed is:
1. A molecular sieve composite membrane comprising:
a) an anodic alumina membrane as a support, said anodic alumina
membrane having a packed array of columnar hexagonal cells with
straight through uniform channels; and
b) a VAPO.sub.4 -5 molecular sieve membrane grown hydrothermally
in situ on said anodic alumina membrane, wherein the crystals of
the molecular sieve membrane are in bundles and aligned uni-directionally
on said anodic alumina membrane support with their c-axis vertical
to the support and their a- and b-axes essentially aligned, and
wherein the VAPO.sub.4 -5 synthesis mixture consists of aluminum
isopropoxide, phosphoric acid, vanadyl sulfate, tripropyl-amine
(Pr.sub.3 N) and water and the reaction mixture has a molar ratio
of 1-20 Pr.sub.3 N:1-2 Al.sub.2 O.sub.3 :1-2 P.sub.2 O.sub.5 :0.05-0.2
V.sub.2 O.sub.5 :200-600 H.sub.2 O.
2. The molecular sieve composite membrane as defined in claim 1
wherein the crystals of the molecular sieve membrane are about the
same length.
3. A molecular sieve composite membrane comprising:
a) an anodic alumina membrane as a support, said anodic alumina
membrane having a packed array of columnar hexagonal cells with
straight through uniform channels; and
b) a CoAPO.sub.4 -5 molecular sieve membrane grown hydro-thermally
in situ on said anodic alumina membrane, wherein the crystals of
the molecular sieve membrane are in bundles and aligned uni-directionally
on said anodic alumina membrane support with their c-axis vertical
to the support and their a- and b-axes essentially aligned and wherein
the CoAPO.sub.4 -5 synthesis mixture consists of aluminum isopropoxide,
phosphoric acid, cobalt sulfate, tripropylamine (Pr.sub.3 N) and
water and the reaction mixture has a molar ratio of 1-20 Pr.sub.3
N:1-2 Al.sub.2 O.sub.3 :1-2 P.sub.2 O.sub.5 :0.5-0.2 CoO:200-600
H.sub.2 O.
4. The molecular sieve composite membrane as defined in claim 3
wherein the crystals of the molecular sieve membrane are about the
same length.
5. A molecular sieve composite membrane comprising:
a) an anodic alumina membrane as a support, said anodic alumina
membrane having a packed array of columnar hexagonal cells with
straight through uniform channels; and
b) a CoAPO.sub.4 -5 molecular sieve membrane grown hydro-thermally
in situ on said anodic alumina membrane, wherein the crystals of
the molecular sieve membrane are in bundles and aligned uni-directionally
on said anodic alumina membrane support with their c-axis vertical
to the support and their a- and b-axes essentially aligned, and
wherein the CoAPO.sub.4 -5
synthesis mixture consists of aluminum isopropoxide, phosphoric
acid, cobalt mitrite, tripropylamine (Pr.sub.3 N) and water and
the reaction mixture has a molar ratio of 1-20 Pr.sub.3 N:1-2 Al.sub.2
O.sub.3 :1-2 P.sub.2 O.sub.5 :0.5-0.2 CoO:200-600 H.sub.2 O.
6. The molecular sieve composite membrane as defined in claim 5
wherein the crystals of the molecular sieve membrane are about the
same length.
7. A molecular sieve composite membrane comprising:
a) an anodic alumina membrane as a support, said anodic alumina
membrane having a packed array of columnar hexagonal cells with
straight through uniform channels; and
b) a CoAPO.sub.4 -5 molecular sieve membrane grown hydro-thermally
in situ on said anodic alumina membrane, wherein the crystals of
the molecular sieve membrane are in bundles and aligned uni-directionally
on said anodic alumina membrane support with their c-axis vertical
to the support and their a- and b-axes essentially aligned and wherein
the CoAPO.sub.4 -5 synthesis mixture consists of aluminum isopropoxide,
phosphoric acid, cobalt sulfate, triethylamine (Et.sub.3 N) and
water and the reaction mixture has a molar ratio of 1-20 Et.sub.3
N:1-2 Al.sub.2 O.sub.3 :1-2 P.sub.2 O.sub.5 :0.05-0.2 CoO:200-600
H.sub.2 O.
8. The molecular sieve composite membrane as defined in claim 7
wherein the crystals of the molecular sieve membrane are about the
same length.
9. A molecular sieve composite membrane comprising:
a) an anodic alumina membrane as a support, said anodic alumina
membrane having a packed array of columnar hexagonal cells with
straight through uniform channels; and
b) a AlPO.sub.4 -5 molecular sieve membrane grown hydro-thermally
in situ on said anodic alumina membrane, wherein the crystals of
the molecular sieve membrane are in bundles and aligned uni-directionally
on said anodic alumina membrane support with their c-axis vertical
to the support and their a- and b-axes essentially aligned, and
wherein the AlPO.sub.4 -5 synthesis mixture consists of aluminum
isopropoxide, phosphoric acid, tripropylamine (Pr.sub.3 N) and water
and the reaction mixture has a molar ratio of 1-20 Pr.sub.3 N:1-2
Al.sub.2 O.sub.3 :1-2 P.sub.2 O.sub.5 :200-600 H.sub.2 O.
10. The molecular sieve composite membrane as defined in claim
9 wherein the crystals of the molecular sieve membrane are about
the same length.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel molecular sieve composite
membrane, more particularly to a composite membrane comprising an
anodic alumina membrane as a templating support, as well as the
vertically-oriented and closely-packed molecular sieve crystals
grown in situ on anodic alumina membrane by a hydrothermal method,
any pre- or post-treatments and modifications of substate is not
required. This tailor-made synthesis is initiated by crystal growth
in a geometrically confined environment, i.e., 200-nm-pore-diameter
cylindrical channels of the support.
2. Description of the Prior Art
Crystalline zeolite and zeolite-like molecular sieves are aluminosilicate-
and aluminophosphate-based materials with uniform intra-framework
cages and channels of microporous or mesoporous size. With tailor-made
acidity, specific pore structure and high thermal stability, they
have been used as a unique type of catalysts for traditional, shape-selective
chemical reaction of guest hydrocarbon molecules, especially in
petroleum refining and petrochemical industry. Recently, attention
has focused on the promising applications where molecular sieve
crystals can serve as a potential nanometer or smaller sized host
for the manufacture of advanced materials such as organized metal
clusters, metal oxides or metal sulfides, conducting polymers, and
trapped supra-molecular compounds that exhibit specific optic, optoelectronic
and electrochemical properties and endow for devices of molecular
wires, quantum electronics and nonlinear optics (Science, 263 1698
(1994), Adv. Mater., 4 612 (1992)).
To control exquisitely over chemical reactions and improve significantly
the production of devices on the smallest scale, a technological
challenge now is to be able to produce the zeolites or zeolite-like
molecular sieves in the form of giant perfect single crystals or
high quality thin films, rather than tiny powder forms prepared
under conventional synthetic conditions. However, the synthesis
of large single crystals is limited, the elaborate preparation of
a continuous polycrystalline zeolite membranes has thus attracted
much attention.
Much effort has been put into the preparation of molecular sieve
membrane, for example: synthesis of composite zeolite films by embedding
zeolite crystals in plastic materials (J. Membrane Sci., 73 119
(1992)) and the in situ growth of zeolites on porous ceramic substrates
by E. R. Geus et al. (J. Chem. Soc. Faraday Trans., 88 3101 (1992),
J. Membrane Sci., 82 15 (1993), J. Chem. Soc., Chem. Commum., 339(1994),
Microporous Mater. 3 565 (1995)). The performance of these films
is not quite satisfactory due to either the insufficient formation
of zeolite crystals or the substrate having low porosity. Zeolite
films with/without cellulose moulding were synthesized by T. Sano
et al.(Zeolites, 11 842 (1991), J. Chem. Soc., Chem. Commun., 2087
(1994)), these zeolite films were found to be either out of control
in shape or lack of the mechanical strength.
Besides shape-selective catalysis and adsorption, the molecular
sieve membranes can be used as hosts to orient guest atom cluster
and molecules and can be considered as the promising template for
sensor, conductor and non-linear optical materials. The advantage
of preferred orientation of the guest molecules can be macroscopically
improved by aligning the molecular sieve host crystals. Furthermore,
it is also noteworthy that the use of this type of zeolite membranes
on reaction and separation may have better performance than traditional
zeolite powder and may lead to the new applicable industrial processes.
However, preparation of a continuous film of controlled-orientation
molecular sieve crystals on a porous substrate seems difficult.
Increasing attempts have been made to develop aligned molecular
sieve crystals, especially in a membrane form. For instance, pre-grown
crystals of molecular sieves MFI and AFI were uni-directionally
oriented by an electric field and then fixed by an epoxy resin film
(Adv. Mater., 7 711 (1995)); crystals of zinco-(or alumino-) phosphate
molecular sieves were grown along(111) planes on gold surface that
has been modified with organophosphate multilayer films (Science,
265 1839 (1994), Nature, 368 834 (1994)); crystals of MFI were
observed to be ordered by mean of properly treated microstructured
surfaces (Adv. Mater., 8 137 (1996)); films of aligned mesoporous
zeolite were prepared on the freshly cleaved mica(Nature, 379 703(1996))
or at the air-water interface(Nature, 381 589(1996)). Most of these
methods needed the pre- or post-treatments and modifications of
substrate.
SUMMARY OF THE INVENTION
A novel method, for preparing the aligned molecular sieve composite
membrane, has now been reported. The anodic alumina, with uniform
200-nm-pore-diameter straight channels and high porosity (about
30.about.50%), was used as the support for the in situ growing the
oriented molecular sieve crystals. Because of its specific pore
structure, compact nature, as well as thermal and chemical stability,
anodic alumina has been used as the support of sufficient mechanical
strength to resist the stress induced by molecular sieve growth,
and abundant cylindrical pores and OH function groups to accelerate
the rate of molecular sieve synthesis.
Synthesis conditions such as temperature, dilution degree of solution,
chemical composition (especially, template concentration) and reactant
source will influence the crystal nucleation rate and transportation
of nutrients for the synthesis of zeolite through the nanometer-sized
channels of substrate. By controlling these factors, the oriented
growth of transitional metal containing aluminophosphate crystals
in bundles are achieved, not only with their c-axes vertical to
the substrate, but their a- and b-axes also aligned to a substantial
extent.
BRIEF DESCRIPTION OF THE DRAWINGS
Tab. 1 is the structural parameters of AFI and MFI
Tab. 2 is the structural parameters of FAU and MCM-41
FIGS. 1(a)-1(b) are SEM image of the aligned molecular sieve VAPO.sub.4
-5 crystals grown on anodic alumina.
FIGS. 2(a)-2(b) are SEM image of the aligned molecular sieve CoAPO.sub.4
-5 crystals grown on anodic alumina.
FIGS. 3(a)-3(b) are SEM image of the aligned molecular sieve AlPO.sub.4
-5 crystals grown on anodic alumina.
FIGS. 4(a)-4(b) show VAPO.sub.4 -5 crystals anchored inside the
straight channel walls of anodic alumina membrane.
FIGS. 5(a)-5(c) are Scanning electron micrographs (top view) of
VAPO.sub.4 -5/anodic alumina composite membrane.
a) SEM micrograph of VAPO.sub.4 -5 crystals with different length
and with a- or b-axis preferred orientation.
b) SEM micrograph of VAPO.sub.4 -5 crystals with same length and
with a- or b-axis preferred orientation in certain extent.
c) SEM micrograph of VAPO.sub.4 -5 crystals with a- or b-axis random-aligned
morphology.
FIG. 6 is a XRD pattern of aligned molecular sieve VAPO.sub.4 -5
crystals grown on anodic alumina.
FIG. 7 is a XRD pattern of aligned molecular sieve CoAPO.sub.4
-5 crystals grown on anodic alumina.
FIG. 8 is a XRD pattern of aligned molecular sieve AlPO.sub.4 -5
crystals grown on anodic alumina.
FIG. 9 is a Proposed synthetic scheme for the vertically-aligned
molecular sieves grown on anodic alumina membrane.
a) The structure of anodic alumina membrane.
b) Crystal nucleation inside the cylindrical channels of anodic
alumina membrane.
c) Hexagonal crystals of AFI (VAPO.sub.4 -5 CoAPO.sub.4 -5 or
AlPO.sub.4 -5) grown on the surface of anodic alumina membrane.
DETAILED DESCRIPTION OF THE INVENTION
The aligned molecular sieve composite membrane of the present invention
includes an anodic alumina membrane as a suppport, and the uni-directionally
oriented molecular sieve crystals grown in situ and in bundles on
the anodic alumina membrane.
The anodic alumina membrane used in the present invention can be
prepared from anodization of aluminium metal or aluminum alloy.
The anodization process is frequently completed by reacting an aluminium
metal or aluminum alloy with phosphoric acid or with a phosphoric
acid-containing solution. Anodic porous alumina with channels of
diameter around 200 nm and density about 10.sup.9 -10.sup.10 holes
per cm.sup.2 was prepared by anodic oxidation of aluminum in an
aqueous phosphoric acid mixture. The well-prepared anodic porous
alumina membrane(thickness of .about.50 .mu.m) is of the typical
self-organized fine structure with a packed array of columnar hexagonal
cells, straight-through and uniform channels. Furthermore, the obtained
anodic alumina membrane can be modified by a heat treatment, thermochemical
treatment or chemical surface treatment. Use of an anodization process
of an aluminium metal or aluminum alloy to yield anodic alumina
membrane is a well-established technique and has been discussed
extensively in the literature (J. Electrochem. Soc., 100 411 (1953),
Electrochimica Acta., 23 127 (1978), ibid., 23 135(1978)). However,
the porous anodic alumina membrane has never been used as the support
for growing molecular sieve composite membranes.
The aligned molecular sieve crystals of the present invention are
hydrothermally grown in situ on the anodic alumina support. Molecular
sieves may have channel- or caged-typed micropore structure. Examples
of the molecular sieves having one-dimensional channel-typed micropore
structure include AlPO.sub.4 -5 AlPO-11 VPI-5 mordenite and Nu-10.
Examples of the molecuular sieves having two-dimensional channel-typed
micropore structure include ZSM-5 and silicalite. Examples of the
molecular sieves having three-dimensional cage-typed micropore structure
include zeolite A, zeolite X and zeolite Y.
The crystalline aluminophosphate-based AlPO.sub.4 -5 (AFI) molecular
sieve(space group P6cc) consists of alternating alumina and phosphate
tetrahedra connected into 12-membraned-ring channels (diameter of
7.3 .ANG.) parallel to c-axis. Without a net charge, it is not expected
to have balancing cations in framework void space and ion-exchange
capacity. However, the isomorphic substitution of Al.sup.3+ by Co.sup.2+
(CoAPO.sub.4 -5) and P.sup.5+ by V.sup.4+ (VAPO.sub.4 -5) will generate
the Br.o slashed.nsted acidity and ion-exchange capacity in the
originally neutral aluminophosphate molecular sieves. These properties
may endow the composite membranes with catalytic ability (J. Chem.
Soc. Faraday Trans., 88 2949 (1992)).
Growing the molecular sieve membrane in situ on the anodic alumina
membrane invoves immersing the anodic alumina membrane in a solution
containing a synthetic mixture which can be reacted to obtain molecular
sieve. The growing is performed in a Teflon-lined stainless-steel
autoclaves without stirring under autogenous pressure at 150-190.degree.
C. for 1-2 days. For the molecular sieve VAPO.sub.4 -5 the synthetic
mixture may consist of aluminium isopropoxide (or pseudoboehmite),
phosphoric acid, vanadyl sulfate, tripropylamine (Pr.sub.3 N) and
de-ionized water. The reaction mixture of VAPO.sub.4 -5 has a molar
ratio of 1.0.about.6.0 Pr.sub.3 N: 0.95-1.0 Al.sub.2 O.sub.3 : 0.95-1.0
P.sub.2 O.sub.5 : 0.05-0.1 V.sub.2 O.sub.5 : 200.about.600 H.sub.2
O. For the molecular sieve CoAPO.sub.4 -5 the synthetic mixture
may consist of aluminium isopropoxide (or pseudoboehmite), phosphoric
acid, cobalt sulfate, tripropylamine (Pr.sub.3 N) and de-ionized
water. Optionally, the synthetic mixture for growing CoAPO.sub.4
-5 may include triethylamine (Et.sub.3 N) and cobalt nitrite. The
reaction mixture of CoAPO.sub.4 -5 has a molar ratio of 1.0.about.6.0
Pr.sub.3 N (or Et.sub.3 N): 0.95-1.0 Al.sub.2 O.sub.3 : 0.95-1.0
P.sub.2 O.sub.5 : 0.1-0.2 CoO: 200.about.600 H.sub.2 O. For the
molecular sieve AlPO.sub.4 -5 the synthetic mixture may consist
of aluminium isopropoxide (or pseudoboehmite), phosphoric acid,
tripropylamine(Pr.sub.3 N) and de-ionized water. The reaction mixture
of AlPO.sub.4 -5 has a molar ratio of 1.0.about.6.0 Pr.sub.3 N:
0.95-1.0 Al.sub.2 O.sub.3 : 0.95-1.0 P.sub.2 O.sub.5 : 200.about.600
H.sub.2 O.
After the growth of the molecular sieve membrane on the anodic
alumina membrane is complete, the obtained molecular sieve composite
membranes were washed with de-ionized water under ultrasonic vibration
for several times and dried at 100.degree. C. for 1 hour. The samples
were further calcined at 500-550.degree. C. for 18-72 hours to remove
the organic base occluded in the channels of molecular sieve.
The growth of VAPO.sub.4 -5 CoAPO.sub.4 -5 or AlPO.sub.4 -5 molecular
sieve membrane is perpendicular to the anodic alumina support, and
the integrities of the anodic alumina was still maintained. The
aligned molecular sieve crystals in bundles on the anodic alumina
are grown a close packing layer covering the support (as shown in
following FIG. 1 FIG. 2 and FIG. 3). As shown in these micrographs,
molecular sieve crystals were found to survive hours of ultrasonic
vibration and they were indeed strongly bonded and were anchored
inside the uniform straight channels of anodic alumina. The width
and length of individual VAPO.sub.4 -5 crystal are 0.6-1.8 .mu.m
and 6-8.4 .mu.m, respectively(as shown in FIG. 1a). The area of
close packing of VAPO.sub.4 -5 crystals can be up to 80.mu.m.times.40.mu.m
(as shown in FIG. 1b). As for CoAPO.sub.4 -5 the width of each
crystal is approximated 4.5-9.mu.m (FIG. 2a) and the closely packing
area can be even up to 626.mu.m.times.460.mu.m (FIG. 2b). Hexagonal
AlPO.sub.4 -5 molecular sieve, which are more easily formed as large
crystals, exists isolated or in bundles with length of .about.30.mu.m
and width of .about.20.mu.m on anodic alumina support (FIG. 3).
The direct attachment between the molecular sieve crystals and
the anodic alumina can be demonstrated, as shown in SEM micrographs
of FIG. 4. These hexagonal crystals were found to grow with their
one-dimensional channels almost parallel (with few titling angle)
to the nanometer-sized cylindrical arrays of anodic alumina support.
In the early stage of synthesis, AFI crystals have attached on the
surface of porous anodic alumina substrate and strongly bounded
to and anchored inside the channels of anodic alumina membrane (as
shown in FIG. 4a). Afterwards, they grow further into large and
vertically aligned AFI crystals (as shown in FIG. 1 2 and 3).
VAPO.sub.4 -5(CoAPO.sub.4 -5 or AlPO.sub.4 -5 not shown here)
crystals, with varied length (FIG. 5a) or uniform length (FIG. 5b),
are all grown with their c-axes oriented perpendicular to the anodic
alumina surface, and with their a- and b-axes also orderly aligned
to certain degree. By varying the distance between the substrate
and the surface of reactant mixture, or the compositions of the
reactant gel, the orientation of a- and b-axes of surface-grown
crystals may not exist any more (FIG. 5c).
The composite membranes obtained in all of our experiments exhibit
powder X-ray diffraction (XRD) patterns of the AFI structure. Some
of membrane samples were also ground and examined by powder XRD.
XRD patterns of the surface-grown VAPO.sub.4 -5 crystals (FIG. 6),
CoAPO.sub.4 -5 crystals (FIG. 7) and AlPO.sub.4 -5 crystals (FIG.
8) are compatible with that of AFI material (simulated from structural
parameters reported in literature (ACS Symp. Ser., 218 109 (1983)),
and show a strongly enhanced intensity of the (002) peak at a Bragg
angle 2 .theta. of 20.88.degree. 21.16.degree. and 20.92.degree.,
respectively. This enhanced intensity confirms the preferred vertical
c-axis orientation of the hexagonal crystals.
The incorporation of vanadium and cobalt into AFI framework structure
has been probed by electron spin resonance (ESR) spectroscopy. The
ESR spectrum of as-prepared greenish VAPO.sub.4 -5 composite membrane
is anisotropic with g and splitting parameters characteristic of
atomically dispersed and immobile V.sup.4+ ions. ESR spectrum of
the synthetic blue CoAPO.sub.4 -5 composite membrane shows the resonance
with g.vertline..vertline..about.4.4 g.perp..about.1.9 indicating
framework Co.sup.2+ in a distorted tetrahedral environment with
presumably a high-spin state.
The proposed synthetic scheme of preparing vertically aligned zeolite
crystals on a horizontally placed anodic alumina support is shown
in FIG. 9. The vertically oriented growth may be related to nuclei
distributed inside the cylindrical and straight channels of anodic
alumina substrate (FIG. 9b). The nucleation may occur either inside
the channels (heterogeneous nucleation) or in the solution (homogeneous
nucleation), then transfering to the channels. Nutrients can penetrate
into the channels of 200-nm-pore diameter from reaction mixtures
through capillary condensation and they interact with OH groups
on the wall of channels to participate in the crystallization of
molecular sieve. IR spectra of the anodic alumina of the present
invention shows that the anodic alumina contains OH function group,
with frequency of 900.about.1160 cm.sup.-1 which may participate
in the molecular sieve synthesis. In addition, Sano
et al. (J. Mater. Chem., 2 141 (1992)) and Valtchev et al. (J.
Chem. Soc., Chem. Commun., 2087 (1994)) have also suggested similar
type of interaction between OH groups of cellulose and aluminosilicate
species during zeolite nucleation. If the size of nuclei formed
in the solution is less than the pore diameter of channel (about
200 nm), they may diffuse into the channels of support. The nanocrystals
of AlPO.sub.4 -5(4 20 and 70 nm sized globular particles) were
observed by Caro et al. (Adv. Mater., 7 711 (1995)). The nuclei
inside the channels further grow into large crystals with their
c-axis preferred parallel to the channels, as depicted in FIG. 9c.
In this study, not only the c-axis (i.e., [001] direction) of these
crystals was found to be vertical to the substrate, the a- and b-axes
were also observed to be aligned to a substantial extent.
By using the anodic alumina as the support, the orientation of
the molecular sieve membrane can easily be controlled. As compared
with the previous approaches, any pre- or post-treatments (such
as fixing of aligned crystals by epoxy resin) and modifications
of substrate(such as coating organophosphate multilayer films on
Au support) is not needed. In addition, the obtained aligned molecular
sieve membrane of the present invention is also better than other
aligned molecular sieve composite membrane with a porous ceramics,
silicon wafer or metal as the support. The anodic alumina possesses
cylindrical and straight-through uniform 200-nm-pore-diameter channels
with high porosity (about 30.about.50%) relative to porous ceramics
of high resistance and low throughput, because of their nonstraight-through,
low density and irregular pores. With excellent throughput, the
anodic alumina still has the similar thermal stability as the other
sintered porous alumina. Although the molecular sieve composite
membrane of the present invention is heated to 500.degree. C. to
remove the organic base occluded in the channels of molecular sieves,
the geometries of the cylindric pore and straight-through of anodic
alumina membrane, as well as the surface morphology of the molecular
sieve crystals are still unchanged. Moreover, the cracks, perhaps
induced by thermal stress are not observed in the molecular sieve
membrane. The molecular sieve phase is still strongly bounded and
anchored into the anodic alumina after heat treatment.
In conclusion, the closely packed molecular sieve membrane has
first successfully been in situ grown on the anodic alumina membrane
support. More particularly, these surface-grown and hexagonal-shaped
molecular sieve crystals are well aligned with their c-axes vertical
to the anodic alumina suppport. The molecular sieve composite membrane
of the present invention has the advantages of sufficient mechanical
strength, thermal and chemical stability. Furthermore, the anodic
alumina membrane has uniform cylindrical straight channels with
high density. This allows the molecular sieve phase to be anchored
into the channels (or pores) of the anodic alumina, and the molecular
sieve phase to be strongly bounded to the anodic alumina support.
In addition, the orientation of the molecular sieve layer can be
controlled by the application of the anodic alumina. |